The present invention relates to positioning tape with respect to a rotating head of helical scan or rotating head digital signal recorder. In particular, it provides for precise calibration and a high degree of noise rejection in a tape positioning apparatus.
In rotating head type magnetic tape recorders, a cylindrical mandrel or drum includes a rotating head adapted for transducing engagement with a tape helically wound on such mandrels. Included in many such recorders are automatic tape threading, precise guiding, high-speed searching and step-by-step motion as set forth in the documents incorporated by reference.
In those recorders, stepping from one helical track or stripe to another, having digital data recorded in the stripe, requires a precise stable stepping control. Such is particularly true when the tape is to be recorded on one digital signal recorder and later read by another digital signal recorder.
In such recorders, a data cartridge can be automatically loaded into the data recording device (DRD) which includes a rotating head digital signal recorder. The tape is automatically threaded and would onto a take-up spool or capstan in a known manner. In some of such recorders, a tachometer on the take-up spool or capstan driving motor is used to control the tape motion across the mandrel and past the rotating head. When the tape is first wound on the take-up capstan, an incremental angular motion corresponds to a first shortest step of the tape past the rotating head. As tape is wound onto the capstan and the radius of the outside wrap increases, the same angular step results in an increasing length of tape being transported past the head. Accordingly, if the take-up capstan tachometer is to be used for positioning control, as is desired (without limitation) in connection with practicing the present invention, the angular step must be translated into an appropriate length of tape for insuring uniform stripe-to-stripe distances. Additionally, when stripes are recorded on one recorder and read on another, certain perturbations in the recording can cause varying spacing between successive ones of said stripes. Such variations can be extremely small. Yet, cummulative errors resulting from stepping past a plurality of such stripes in recording or reading data can result in recording errors. Accordingly, it is desired to provide a data recording device which accommodates perturbations in stripe-to-stripe spacing as well. The changes in linear transition of the tape as a tape wrap on a take-up capstan changes.
Most helical scan recorders operate with record tapes that have longitudinally extending transversely sensed servo tracks. Such servo tracks are used in connection with precisely locating the tape with respect to the rotating head for transducing operations. As the rotating head begins its scan of the tape, it crosses over one edge of the tape. Such crossing can induce perturbations in the tape resulting in less than desirable transducing relationships between the head and the record tape. Such perturbations in the tape may result in erroneously interpreted servo data from such tracks. However, as the head completes its scan of the tape and crosses the opposite edge, thereof, fewer perturbations are introduced into the tape. However, because of contamination on the tape or void spots in the magnetic coding, servo errors of a permanent or transitory type can be introduced into the mechanism to position the tape such that the head path is not precisely centered on a given record stripe or track. It is highly desirable in a digital signal recorder where all signals must be successfully recorded and recovered as opposed to video recorders wherein one stripe can be ignored without disastrous effects that the controlling servo mechanism be immuned to noise and jitter for insuring reliable and precise stepping control of the tape past the transducing station.